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The Cambrian “Explosion”, Part 3

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December 17, 2010 Tags: History of Life

Today's entry was written by Keith Miller. Please note the views expressed here are those of the author, not necessarily of The BioLogos Foundation. You can read more about what we believe here.

The Cambrian “Explosion”, Part 3

The Fossil Record: Is there enough evidence ?

There are two opposite errors which need to be countered about the fossil record: 1) that it is so incomplete as to be of no value in interpreting patterns and trends in the history of life, and 2) that it is so good that we should expect a relatively complete record of the details of evolutionary transitions within all or most lineages.

What then is the quality of the fossil record? It can be confidently stated that only a very small fraction of the species that once lived on Earth have been preserved in the rock record and subsequently discovered and described by science.

There is an entire field of scientific research referred to as "taphonomy" -- literally, "the study of death." Taphonomic research includes investigating those processes active from the time of death of an organism until its final burial by sediment. These processes include decomposition, scavenging, mechanical destruction, transportation, and chemical dissolution and alteration. The ways in which the remains of organisms are subsequently mechanically and chemically altered after burial are also examined -- including the various processes of fossilization. Burial and "fossilization" of an organism's remains in no way guarantees its ultimate preservation as a fossil. Processes such as dissolution and recrystallization can remove all record of fossils from the rock. What we collect as fossils are thus the "lucky" organisms that have avoided the wide spectrum of destructive pre- and post-depositional processes arrayed against them.

Soft-bodied organisms, and organisms with non-mineralized skeletons have very little chance of preservation under most environmental conditions. Until the Cambrian nearly all organisms were soft-bodied, and even today the majority of species in marine communities are soft-bodied. The discovery of new soft-bodied fossil localities is always met with great enthusiasm. These localities typically turn up new species with unusual morphologies, and new higher taxa can be erected on the basis of a few specimens! Such localities are also erratically and widely spaced geographically and in geologic time.

Even those organisms with preservable hard parts are unlikely to be preserved under "normal" conditions. Studies of the fate of clam shells in shallow coastal waters reveal that shells are rapidly destroyed by scavenging, boring, chemical dissolution and breakage. Occasional burial during major storm events is one process that favors the incorporation of shells into the sedimentary record, and their ultimate preservation as fossils. Getting terrestrial vertebrate material into the fossil record is even more difficult. The terrestrial environment is a very destructive one: with decomposition and scavenging together with physical and chemical destruction by weathering.

The potential for fossil preservation varies dramatically from environment to environment. Preservation is enhanced under conditions that limit destructive physical and biological processes. Thus marine and fresh water environments with low oxygen levels, high salinities, or relatively high rates of sediment deposition favor preservation. Similarly, in some environments biochemical conditions can favor the early mineralization of skeletons and even soft tissues by a variety of compounds (eg. carbonate, silica, pyrite, and phosphate). The likelihood of preservation is thus highly variable. As a result, the fossil record is biased toward sampling the biota of certain types of environments, and against sampling the biota of others.

In addition to these preservational biases, the erosion, deformation and metamorphism of originally fossiliferous sedimentary rock have eliminated significant portions of the fossil record over geologic time. Furthermore, much of the fossil-bearing sedimentary record is hidden in the subsurface, or located in poorly accessible or little studied geographic areas. For these reasons, of those once-living species actually preserved in the fossil record, only a small portion have been discovered and described by science. However, there is also the promise of continued new and important discovery.

The forces arrayed against fossil preservation also guarantee that the earliest fossils known for a given animal group will always date to some time after that group first evolved. The fossil record always provides only minimum ages for the first appearance of organisms.

Because of the biases of the fossil record, the most abundant and geographically widespread species of hardpart-bearing organisms would tend to be best represented. Also, short-lived species that belonged to rapidly evolving lines of descent are less likely to be preserved than long-lived stable species. Because evolutionary change is probably most rapid within small isolated populations, a detailed species-by-species record of such evolutionary transitions is unlikely to be preserved. Furthermore, capturing such evolutionary events in the fossil record requires the fortuitous sampling of the particular geographic locality where the changes occurred.

Using the model of a branching tree of life, the expectation is for the preservation of isolated branches on an originally very bushy evolutionary tree. A few of these branches (lines of descent) would be fairly complete, while most are reconstructed with only very fragmentary evidence. As a result, the large-scale patterns of evolutionary history can generally be better discerned than the population-by-population or species-by-species transitions. Evolutionary trends over longer periods of time and across greater anatomical transitions can be followed by reconstructing the sequences in which anatomical features were acquired within an evolving branch of the tree of life.


Keith Miller is research assistant professor of geology at Kansas State University in the United States. He is editor of Perspectives on an Evolving Creation (Eerdmans, 2003), an anthology of essays by prominent evangelical Christian scientists who accept theistic evolution. He is also a member of the executive committee of the American Scientific Affiliation, an association of Christians in the sciences, and a board member of Kansas Citizens for Science, a not-for-profit educational organization that promotes a better understanding of science.

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pds - #44144

December 17th 2010

Good points here, and it raises many problems, for example:  Why have we found so many trilobite fossils and trilobite species (thousands) over many millions of years, but no trilobite ancestors prior to the Cambrian?  Where we would expect to see at least some hard bodied trilobite ancestors, we instead see the soft-bodied Ediacara fauna.  Why?


R Hampton - #44153

December 17th 2010

pds,
Scientists studying the evolution of turtles faced a very similar problem, but that didn’t mean turtles spontaneously evolved a fully complete shell any more than did the trilobites. Two years ago, however, the first transitional fossils were discovered in China (Odontochelys semitestacea). Just goes to show that gaps in the fossil records can not be considered proof of evolutionary gaps.


BioLogos - #44157

December 17th 2010

PDS,

Have you seen this article?


pds - #44161

December 17th 2010

R Hampton,

I did not use the word “proof.”  I draw inferences based on all the data.  Isolated fossils don’t significantly change inferences based on the overall patterns of the fossil record.  The overall pattern of sudden appearance and stasis is very well established.

Biologos,

That link does not seem to work.  What does the article say?


RJS - #44162

December 17th 2010

Fortey, R., Briggs, D. and WILLS, M. (1996), The Cambrian evolutionary ‘explosion’: decoupling cladogenesis from morphological disparity. Biological Journal of the Linnean Society, 57: 13–33. doi: 10.1111/j.1095-8312.1996.tb01693.x


Tim - #44165

December 17th 2010

PDS,

“Isolated fossils don’t significantly change inferences based on the overall patterns of the fossil record.”

If those “isolated” fossils possess transitional features, they most certainly do impact the inferences one draws from the fossil record.  For instance, just a small number of hominid skulls for homo habilis, homo ergaster, homo heidelbergensis, etc. make all the difference in the world in terms outlining a branching pattern of ancestry leading up to humans.  If you found just one or two fossils of a different species, you don’t have to wait until you have a dozen or more before you can declare you’ve identified a new species, and that that new species matters.

I don’t see how quantity comes into this.


R Hampton - #44190

December 17th 2010

pds,
...and the inference you would have drawn from the limited turtle fossils would have been a sudden evolutionary appearance of a fully developed shell some 210 mya - that is, until the discovery of a transitional just two years ago. Do you see the parallel? Absence of evidence is not evidence of absence.


Glen Davidson - #44200

December 18th 2010

It’s always instructive to try to think of why a number of phyla arose rapidly near the beginning of the Cambrian period, and so many died off soon after.  The exact design reasons for this, and the “designer” never bothering to consult the old blueprints again (most really good things made by humans can be completely annihilated and yet reappear) in producing later life forms, eludes everybody.  Creationists know to ignore the problem, however, and since they aren’t really out to explain anything at all they are not troubled by their lack of explanation.

The rest of the fossil record is essentially the same.  Relative dating worked prior to the acceptance of evolutionary theory, although there certainly was no design reason for why exact copies of extinct organisms could not reappear after a time.  Evolution gave us the reason that relative dating—contingency won’t allow for exact replication of contingent features (or anyhow, the odds are heavily against).

But that’s what science does, it explains features.  Creationism tries to explain away some, while ignoring many more.  It never really explains anything.

Glen Davidson


Glen Davidson - #44201

December 18th 2010

Hm, dropped a word.  With correction in brackets, it should be:

Evolution gave us the reason that relative dating works…

Glen Davidson


pds - #44258

December 18th 2010

RJS and “Biologos”,

Thanks.  The link seems to have been fixed.  From the abstract, I don’t see how it resolves the questions raised in my first comment.  Can Biologos explain?  Does it propose a solution to the trilobite fossil problem that is accepted by a consensus of paleontologists?

Tim,

I am talking about the huge number of trilobite fossils, and the lack of any ancestors.  Do you want to address that problem?

Glen,

Your question “Why would the designer do it that way?” is an interesting philosophical question.  Perhaps “designer psychology” will be a new field of study.  I am not sure if you intended it as an explanation of the trilobite problem I outlined.


BioLogos - #44277

December 18th 2010

PDS #44258

“From the abstract, I don’t see how it resolves the questions raised in my first comment.  Can Biologos explain?”

The abstract speaks for itself, pds, but it would likely be good for you to get a hold of the article and read it.  We would love for you to write a summary of it after you have done so.

“Does it propose a solution to the trilobite fossil problem that is accepted by a consensus of paleontologists?”

Yes it proposes a very interesting solution.  However, the specific answers to questions surrounding the origin of trilobites, like so many scientific questions, are still up in the air.  Dr. Miller does a good job in this essay of giving a nut-shell explanation of some of the issues.


John - #44280

December 18th 2010

pds:
“I draw inferences based on all the data.”

What inferences do you draw from the terabytes of sequence available? Is there a lack of continuity in protein families between phyla?

And if you try to describe this as mere “similarity,” you are emphatically ignoring vast amounts of data.

“Isolated fossils don’t significantly change inferences based on the overall patterns of the fossil record.”

And what do you make of the overall patterns of protein families across phyla?


Glen Davidson - #44281

December 18th 2010

Your question “Why would the designer do it that way?” is an interesting philosophical question.

It’s a crucial science question if you’re going to do designer science.  As you have no answer for why any designer would do it that way, it’s obvious that you’re no more interested in actually doing science than is any other IDist.

  Perhaps “designer psychology” will be a new field of study.

I should think that would be something anyone doing design science would be keenly interested in doing, as it would be crucial for answering design questions.  The exceptional lack of interest in such questions from IDists indicates just how unscientific ID is.

  I am not sure if you intended it as an explanation of the trilobite problem I outlined.

Actually, I brought it up such issues because science has answers to a host of issues, and questions surrounding a rather few.  Not being interested in science answers, you choose to focus upon the latter, even though you have no answers beyond “poof”.  It’s not like I didn’t indicate as much, but you would rather ignore the ID problems yet again.

Glen Davidson
.


beaglelady - #44296

December 18th 2010

Hi Glen,

What part of “poof” didn’t you understand?


Tim - #44299

December 18th 2010

PDS,

Hampton stated the following:

“Scientists studying the evolution of turtles faced a very similar problem, but that didn’t mean turtles spontaneously evolved a fully complete shell any more than did the trilobites. Two years ago, however, the first transitional fossils were discovered in China (Odontochelys semitestacea). Just goes to show that gaps in the fossil records can not be considered proof of evolutionary gaps.”

to which you replied:

“Isolated fossils don’t significantly change inferences based on the overall patterns of the fossil record.”

Which is why I posted what I did.


Glen Davidson - #44313

December 18th 2010

What part of “poof” didn’t you understand?

I’m good on the “po.”  The “of” leave me baffled, though, and makes me doubt that it’s true science.

Glen Davidson


Keith Miller - #50652

February 9th 2011

In response to the question of pds about trilobite transitional forms:

One of the characteristics of trilobites that made them such a successful invertebrate group, and gave them such a rich fossil record, is that they had a calcified skeleton unlike most arthropods whose exoskeleton is chitinous.  Transitional forms that would connect trilobites to other arthropods groups would be expected to be found among unmineralized taxa.  Such taxa are largely confined to exceptional fossil deposits such as the Burgess Shale and Chengjiang.  It turns out there there are numerous unmineralized arthropods in these deposits that provide good candidates for sister group and stem group relatives of the trilobites. 

Keith


Keith Miller - #50653

February 9th 2011

Part 2 of my response on trilobites:

Trilobites are members of a larger taxonomic group called the Arachnomorpha.  Some of these appear quite close to the trilobites including the Helmetids, Tegopeltids, Naraoiids, and Xanderellids.  A very small (2-6 mm) primitive arthropod from the Chengjiang called Primicaris has a single undivided carapace.  The Tegopeltids also have a single dorsal shield.  In the Naraoiids the head shield (or cephalon) is separated from the rest of the body.  The Helmetids possess a pygidium and thoracic segments, and have trilobite-like limbs.  This is the group closest to trilobites.  A good source of information on these various trilobite-like arthropods (including references to the published literature) can be found in a website by S.M. Gon III

I talk about the origin of the arthropod phylum in a later installment of this essay.

Keith


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